With Russian unique church improvements, among the upgrades, you can also also send a one time shipment of 9 Kalmucks (Dragoons) and Bashkir’s (Hussars). If you research this, why are you not able to keep training after and using these units after? Once your initial units are gone, that’s it you can’t train these units again.
Was it meant to be just a one time thing for balance? Not an expert here, so that’s why I ask.
Many civilisations have this kind of one-time uniquely-named unit shipment that costs some resources. (Winged Hussars for Germany, Besterios for Portuguese, or church shipments like Stadhouders for Dutch) Those are usually a larger batch compared to other “free” unit shipments. They also often have guard or imperial guard level upgrade.
They are not meant to be trained afterwards, those shipments are meant as a sudden powerful boost into your army. That is why they cost resources unlike other unit shipments.